Auxiliary power units (APUs) are gas turbine engines used in aircraft systems to provide pneumatic and electrical power to the aircraft, primarily when the aircraft is on the ground. This pneumatic and electrical power is provided by the main propulsion engines when the aircraft is in flight. Power transfers are required between the APU and the main engines at the beginning and at the end of aircraft flights. Additionally, electrical power transfers are also required between the APU and ground power supplies.
Current aircraft designs typically operate electrical power systems at a constant frequency. Electrical power transfers for this type of system can either be a “break transfer,” during which neither the APU nor the main engine generator supplies the electrical bus for a short period, or a “no break power transfer,” during which the APU generator temporarily operates in parallel with the main engine of ground power to simultaneously supply electrical power to the aircraft electrical bus.
A no-break power transfer (NBPT) technique may be used to achieve load transfer without interrupting the supply of electrical power to the aircraft. During no-break power transfer, the components of the APU speed (i.e., frequency and phase) and the main engine speed are matched so that the APU and the engine can be connected simultaneously to the same electrical bus. No-break power transfer poses its own problems, however. First, the APU may be required to carry a large electrical load during the frequency matching process, potentially causing the APU to overload and cause high-temperature problems in the APU. The maximum electrical load that the APU can handle varies based on the APU operating conditions, such as air temperature and pressure, APU speed, etc. Electrical overload is particularly a problem if the APU is loaded solely by an electrical load with no pneumatic load that can be reduced to compensate for the increased electrical load.
In aircraft designs operating at fixed speeds, no-break power transfer can be conducted by temporarily changing the engine speed to conduct the transfer with the APU and then returning to the predetermined fixed speed. However, modern aircraft designs are increasingly adopting variable frequency electrical bus networks in place of conventional constant frequency systems. The variable frequency bus causes the no-break power transfer process to occur over a range of frequencies rather than a fixed frequency. This makes successful frequency matching between the APU and the main engine more difficult because the APU and the engine no longer return to a single predetermined fixed speed; instead, the speed can vary over a wide range.
There is a desire for a method and system that can ensure frequency matching between an APU and a main engine in a variable frequency system. There is also a desire for a method and system that can conduct no-break power transfer between an APU and a main engine without imposing excessive electrical loads on the APU.